Observations upon Stoiopora fibrosa, etc. 373 



dependent upon the limits of growth have not been ascertained. So 

 far, however, as the describer's observations have extended, the distinc- 

 tions in the arrangement of the diaphragms afford, apparently, one 

 means for inferring differential structures; those variations implying 

 it is presumed, peculiarities in the form of the polypes. In the 

 remarks upon Chetetes radians, it is shown that the diaphragms are 

 distributed in concentric or parallel bands, and often at considerable 

 distances the bauds being composed of a single series of plates, or of a 

 variable number of closely situated laminai, whilst in the account of 

 Chetetes iietropditamis it is explained that the diaphragms are irregularly 

 distributed over the whole area of a vertical section. In other fossils 

 belonging to the genus, but not included in this notice, distinct pecu- 

 liarities were observed. It is further suggested that the characters 

 exhibited by well preserved, terminal edges of the tubes might afford 

 some aid in specific determinations ; a protected surface of Chetetes 

 radians, which presented probably a periodical renewal of growth, 

 having the edges surmounted by a series of very prominent tubercles. 

 The above remarks are not offered with the idea that the characters 

 alluded to could constitute alone specific distinctions, but they are ad- 

 vanced with a hope that they may assist in discovering essential 

 structures. 



Chetetes petropolitanus. 



Globular, hemispherical, or inversely conical ; tubes polygonal ; irreg- 

 ularly arranged ; diaphragms not in bands, but unequally distributed 

 throughout the tubes. 



Favosites petropolitanus (Pander, 1830). 



Calamopora fibrosa (Eichwald, 1840). 



A. careful examination of specimens, agreeing with M. Pander's 

 figures, and general remarks, led to the inference that they possessed 

 all the essential generic characters of Chetetes. 



M. Pander considers the globular, hemispherical, and conical speci- 

 mens described by him as varieties of one species, and it is believed 

 that the differences which may be noticed internally depend on the 

 conditions under which the polypidom was developed. In a well 

 preserved hemispherical specimen, two inches in diameter, and 9 lines 

 in the thickest part, the tubes did not radiate suddenly from the center, 

 but the lateral increase had been produced by the successive addition 

 of marginal tubes, which sprung from the base and slightly inclined 

 outwai'd. 



The concave under-surface of the same specimen exhibited, besides 



